1. When Biff in alternate 1985 tells Marty the exact circumstances under which he got the almanac, Biff says that he asked the old man (2015 Biff) if there is a catch.
2. Later on, Marty witnesses the 2015 Biff giving 1955 Biff the almanac.
3. 1955 Biff doesn't ask if there is a catch. It just goes on from 2015 Biff proving that it isn't a fake, to him saying that all he (1955 Biff) has to do is bet on the winner and he'll never lose.